FritzBox Fon WLAN 7270 as outbound gateway

Since my ISDN PBX died, I've been using a FritzBox Fon Wlan 7270 as a stop-gap, while I sort out migrating the numbers over to a SIP trunk which will be hosted on 3CX.

I've used the internet telephony feature in the FB to allow it to connect to 3CX, so that I can have some ISDN terminals (and my entry phone) connecting to 3CX. But until today I'd not been able to get 3CX to make outgoing calls through the FritzBox as well, so I thought I'd explain here how I did manage to get it to work.

First, I have the 54.04.76 firmware on the FritzBox; it may work with the later .81 version, but after lots of resets and reconfigs, I'm not going to tinker any more now that it's working.

Also, I'm not using the FritzBox to provide my internet connection; it's essentially in place just as a VoIP gateway. I have a connection via a separate router, and the FB is set up as if it were getting its connection via a cable modem; on the internet connection screen (expert mode) it's set to connect to the net via LAN 1, and the operating mode is set to "Share existing Internet connection in the network (IP Client)"

There are fixed IPs for all the systems on the network, and names in the DNS that match to them, so I can access the FritzBox web interface via fritzbox.mydomain.net for example.

The .76 firmware adds a SIP server to the FB; click to add another telephony device, choose telephone, and LAN/WLAN as the interface. You then see a screen with the extension number (which starts at 620), two boxes to enter the password, and the message 'Realm: fritz.box'

If you're using the FB as your broadband router, that's probably all you need to do. But I found that in the config I'm using, I couldn't get 3CX or VoIP phones to register with the Fritz sip server; the 3CX log reported a 404 Not Found error for each registration attempt. I tried putting the realm on the user name, but what actually solved it was using the IP address for the registrar, rather than the FQDN, ie 94.30.xx.yy rather than fritzbox.mydomain.net

So, in 3CX, choose 'Generic SIP provider', use the IP address of the FritzBox for the registrar, 620 as the user id (or whatever else, if you're adding more extensions), and whatever you entered as the password, and then 3CX can register with the FritzBox, giving you a SIP account.

You can then complete the wizard on the FritzBox, choosing which line will be used to make outgoing calls, and to which numbers (if using ISDN) the phone will respond.

When my ISDN is switched off, I'll be able to use the analogue FXO port on the FritzBox to provide myself with emergency fallback that way; the two other FXS ports allow me to link my entry phone and other handsets (and the ISDN handsets) to 3CX.

So, finally with the .76 firmware, as long as you remember to use the IP address of the FritzBox in the VoIP provider configuration for 3CX, you can use one of these FritzBoxes as a bidirectional VoIP gateway (or more directions that that, if you consider it's providing links to ISDN, PSTN and SIP); pretty good value for the price really, and ensures I'll have emergency call support via PSTN, plus fallback if the SIP trunks go down, when I finally switch entirely to VoIP.